H1B Visa Stamping - Dropbox

My H1B petition(I797A) and H1B Visa are valid until 01/18/2019. I recently got my H1B extension petition approved with the validity from 01/19/2019 to 12/27/2021. I am planning to get my Visa stamped in India so that I get my H1B visa until 12/27/2021 in the passport. if the stamping gets done successfully and i get visa stamped till 12/2021, what will happen to the current unexpired visa?. would there be a case where they cancel out my current unexpired visa which is valid until 18th jan 2019 and i would end up in a situation where i may not be able to travel to US until 19th jan 2019 which is the start date of my latest petition. Please clarify and suggest accordingly.

No, none will be cancelled. Your new visa validity would be until Dec 2021. Both the petition numbers will be mentioned on your new visa stamp, indicating the first approval to Jan 2019 and the second petition number with validity until Dec 2021.

Thanks ramanan256 for the reply. i am eligible for dropbox option and i would have to carry both my active I797A documents correct?. Do i need to inform them that i will be returning to US before the start date of the extended petition or will they take care by seeing the documents. Please clarify. I would not want to end up in a situation where i may not be able to travel US until 19 Jan 2019 which is the start date of latest petition.